Humidification-dehumidification (HDH), which is shown schematically in FIG. 1, is a desalination process that uses air as a carrier gas to evaporate water from a saline feed stream. This HDH system 10 includes a humidifier 12 and a dehumidifier 14. In the humidifier 12, a warm liquid feed humidifies a cool, dry carrier-gas (e.g., air) stream in counterflow. The warm, moist carrier-gas stream exits the humidifier and is cooled and dehumidified in the dehumidifier 14, producing a pure water condensate. A liquid-feed conduit 16 circulates the liquid feed through the system, while a carrier-gas conduit 18 circulates the carrier-gas stream through the system.
The cooled carrier gas is either recirculated in a closed loop (closed-air cycle) via conduit 18 or exhausted to the surroundings (in an open-air cycle). HDH can be particularly advantageous (e.g., in comparison with reverse osmosis) for small-scale applications, for high-salinity feeds, and where the feed liquid presents a high scaling potential. HDH systems are further described in U.S. Pat. No. 8,292,272 B2; U.S. Pat. No. 8,252,092 B2; and U.S. Pat. No. 8,647,477 B2.
In pressure-retarded osmosis (PRO), a concentrated (higher-salinity) draw stream and a lower-salinity feed stream are separated by a semi-permeable membrane in counterflow. The semi-permeable membrane selectively admits water but rejects dissolved salts. The difference in osmotic pressure on either side of the membrane causes a flow of pure water from the feed stream into the pre-pressurized draw, thereby increasing the volumetric flow rate of the pressurized draw stream, which can be depressurized in a turbine to produce electrical work. Apparatus and methods for pressure-retarded osmosis are described in U.S. Pat. No. 3,906,250 (S. Loeb, Univ. Ben Gurion). An apparatus referred to as a salinity gradient power unit (SGPU) is described in U.S. Pat. No. 8,695,343 B2 (N. Moe, General Electric Co.).
In brine-recirculation HDH (HDH-BR), which is also known as a “closed-water” cycle, a portion of the concentrated brine from the humidifier is recirculated, cooled, and mixed with the feed. This technique, which can be important in brine concentration applications, is used to achieve higher recovery ratios, which are calculated as RR={dot over (m)}p/{dot over (m)}f, than are obtainable in a single-pass, or open-water HDH arrangement. As the brine and feed streams are mixed, however, exergy is destroyed.